In an internal combustion engine, it is known for a fuel pump to supply fuel to a high-pressure accumulator (or common rail), from which it is delivered into each cylinder of the engine by means of a dedicated fuel injector. Typically, a fuel injector has an injection nozzle which is received within a bore provided in a cylinder head of the cylinder; and a valve needle which is actuated to control the release of high-pressure fuel into the cylinder from spray holes provided in the nozzle.
Historically common rail fuel injectors have opened and closed the needle by way of a hydraulic servo mechanism (e.g. a power assistance), such as that described in EP 0647780 or EP 0740068.
A solenoid-actuated hydraulic servo fuel injector such as that of EP 0740068 is illustrated in FIG. 1. The fuel injector 1 comprises a valve body 3 defining a blind bore 5 that terminates at a nozzle region 7; and an elongate valve needle 9 having a tip region 11 that is slidable within the bore 5, such that the tip 11 can engage and disengage a valve seat 13 defined by an inner surface of the nozzle 7. The nozzle 7 is provided with one or more apertures (or spray holes; not shown) in communication with the bore 5. Engagement of the tip 11 with the valve seat 13 prevents fluid escaping from the valve body 3 through the apertures, and when the tip 11 is lifted from the valve seat 13, fluid may be delivered through the apertures into an associated engine cylinder (not shown).
Although not show clearly in FIG. 1, the valve needle 9 is shaped such that the region that extends between the gallery 15 and the nozzle 7 is of smaller diameter than the bore 5 to permit fluid to flow between the valve needle 9 and the inner surface of the valve body 3. An annular gallery 15 is provided within the valve body 3. The gallery 15 communicates with a fuel supply line 17 arranged to receive high-pressure fuel from an accumulator of an associated fuel delivery system. In order to permit fuel to flow from the gallery 15 towards the nozzle 7, the valve needle 9 is provided with a fluted region 19 which also acts to restrict lateral movement of the valve needle 9 within the valve body 3.
A chamber 21 is provided within the valve body 3 at a position remote from the nozzle 7, the chamber 21 communicating with the high pressure fuel line 17 through a restrictor 23. The chamber 21 is closed by a plate 25. The end of the valve needle 9 remote from the tip 11, is provided with a reduced diameter projection 27, the projection 27 guiding a compression spring 29 which is engaged between the valve needle 9 and the plate 25 to bias the valve needle 9 to a position in which the tip 11 engages the valve seat 13.
A body 31 engages the side of the plate 25 opposite that engaged by the valve body 3, the body 31 and plate 25 together defining a chamber 33 which communicates with the chamber 21 through an aperture 35. The body 31 is provided with a bore within which a valve member 37 is slidable. The valve member 37 comprises a cylindrical rod provided with an axially extending blind bore, the open end of the bore being able to communicate with the chamber 33 when the valve member 37 is lifted such that the end thereof is spaced from the plate 25, such communication being broken when the valve member 37 engages the plate 25. A pair of radially extending passages 39 communicate with the blind bore adjacent the blind end thereof, the passages 39 communicating with a chamber which is connected to a suitable low pressure drain.
The body 31, plate 25 and valve body 3 are mounted on a nozzle holder 41 by means of a cap nut 43. The nozzle holder 41 includes a recess within which a solenoid actuator 45 is provided.
The valve member 37 carries an armature such that upon energisation of the solenoid actuator 45, the armature and valve member 37 are lifted so that the valve member 37 disengages the plate 25. On de-energising the solenoid actuator 45, the valve member 37 returns to its original position under the action of a spring 47.
In use, the valve needle 9 is biased by the spring 29 such that the tip 11 engages the valve seat 13 and, thus, delivery of fuel from the apertures does not occur. In this position, the pressure of fuel within the chamber 21 is high, and hence the force acting against the end of the valve needle 9 due to the fuel pressure, and also due to the resilience of the spring 29 is sufficient to overcome the upward force acting on the valve needle 9 due to the high pressure fuel acting against the angled surfaces of the valve needle 9.
In order to lift the tip 11 of the valve needle 9 away from the valve seat 13 to permit fuel to be delivered from the apertures, the solenoid actuator 45 is energised to lift the valve member 37 against the action of the spring 47 such that the end of the valve member 37 is lifted away from the plate 25. The lifting of the valve member 37 permits fuel from the chamber 33 and hence the chamber 21 to escape to drain through the bore of the valve member 37 and passages 39. The escape of fuel from the chamber 21 reduces the pressure therein, and due to the provision of the restrictor 23, the flow of fuel into the chamber 21 from the fuel supply line 17 is restricted. As the pressure within the chamber 21 falls, a point will be reached at which the force applied to the valve needle 9 due to the pressure within the chamber 21 in combination with that applied by the spring 29 is no longer sufficient to retain the tip 11 of the valve needle 9 in engagement with the valve seat 13, and hence a further reduction in pressure within the chamber 21 will result in the valve needle 9 being lifted to permit fuel to be delivered from the apertures. Typically, a 20% reduction in pressure within the chamber 21 is sufficient to cause the tip 11 of the valve needle 9 to lift from the valve seat 13 and for a fuel injection from the apertures to commence.
In order to terminate delivery, the solenoid actuator 45 is de-energised and the valve member 37 moves downwards under the action of the spring 47 until the open end engages the plate 25. This movement of the valve member 37 breaks the communication between the chamber 33 and the drain and, hence, the pressure within the chamber 33 and chamber 21 will increase. Eventually a point is reached at which the force applied to the valve needle 9 due to the pressure within the chamber 21 and the spring 29 exceeds that tending to open the valve needle 9, and the valve needle 9 will then move to a position in which the tip 11 engages the valve seat 13 to prevent further delivery of fuel.
A solenoid-actuated hydraulic servo mechanism such as that of FIG. 1 means that a low force control valve 37 can be used to switch the high forces on the valve needle 9. With low forces on the control valve 37, a relatively inexpensive and simple solenoid can give a suitably fast enough response in the injector for most purposes. However, a number of disadvantages are associated with the design of such servo injector mechanisms. In this regard, prior art servo designs are subject to a lag period between energisation of the solenoid and commencement of the fuel injection event, during which a parasitic flow of fuel is channelled to a low-pressure fuel drain. Therefore, a hydraulic servo injector cannot always be made to commence a fuel injection event as quickly as may be desired. Moreover, the faster the response desired, the higher the fuel flows required for the hydraulic servo and the higher the resulting parasitic losses from the servo mechanism. The parasitic fuel flow also undesirably returns heat to the fuel supply.
More recently some injectors have used a piezoelectric actuator to directly move the needle (e.g. EP 0995901; EP 1174615). These designs eliminate both the parasitic losses from the servo flows and the time delays in the servo. Some of them also have an accumulator volume within the injector which ensures that maximum pressure is available at the nozzle seat and that wave activity (which could interfere with multiple injections) is minimised.
As illustrated in FIG. 2, a known piezoelectrically actuated fuel injector may comprise a valve body 3 having a blind bore 5 extending into a nozzle region 7 provided with a plurality of apertures (or fuel spray holes; not shown); and a valve needle 9 reciprocable within the bore 5 between injecting and non-injecting positions, as previously described. A piezoelectric actuator stack 49 is operable to control the position occupied by a control piston 51, the piston 51 being moveable to control the fuel pressure within a control chamber 53 defined by a surface associated with the valve needle 9 of the injector and a surface of the control piston 51. The piezoelectric actuator stack 49 comprises a stack of piezoelectric elements, the energisation level, and hence the axial length, of the stack being controlled by applying a voltage across the stack. Upon de-energisation of the piezoelectric stack 49, the axial length of the stack is reduced and the control piston 51 is moved in a direction which causes the volume of the control chamber 53 to be increased, thereby causing fuel pressure within the control chamber 53 to be reduced. The force applied to the valve needle 9 due to fuel pressure in the control chamber 53 is thus reduced, causing the valve needle 9 to lift away from a valve needle seating (not shown) under the influence of high-pressure fuel on surfaces of the valve needle 9, so as to permit fuel delivery into an associated engine cylinder via one or more apertures (or spray holes; not shown).
In order to cause initial movement of the valve needle 9 away from its seating, a relatively large retracting force must be applied to the valve needle 9 to overcome the downwards (closing) force on the valve needle 9. Typically, the large retracting force applied to the valve needle 9 is maintained throughout the opening movement, until the valve needle 9 reaches its full lift position. However, in theory, once valve needle 9 movement has been initiated, a reduced force is sufficient to cause continued movement of the valve needle 9 towards its full lift position. Hence, many known fuel injectors of this type are relatively inefficient as a significant amount of energy is wasted in applying a large retracting force to the valve needle 9 throughout its full range of movement.
To terminate a fuel injection event, the stack 49 is returned to its initial energisation state, and as a result, the piston 51 also returns substantially to its initial position thereby reducing the volume of the control chamber 53. The consequential increase in fuel pressure within the control chamber 53 applies an increased closing force on the valve needle 9, and a point is eventually reached at which the fuel pressure within the control chamber 53 in conjunction with the spring 29 is sufficient to return the needle 9 into engagement with the valve seating (not shown).
In the piezoelectric fuel injector illustrated in FIG. 2, the control piston 51 is part of a hydraulic amplifier system situated between the actuator stack 49 and the needle 9, such that axial movement of the actuator 49 results in an amplified axial movement of the needle 9. In contrast to the fuel injector illustrated in FIG. 2, some piezoelectrically-actuated fuel injectors may be of the type in which energisation (rather than de-energisation) of the piezoelectric stack is required to initiate a fuel injection event.
In addition to the potential faster injector response time of the piezoelectrically operated valve, a further benefit of using a piezoelectric actuator for direct control over the movement of a valve needle is that the axial length of the piezoelectric stack can be variably controlled by changing the amount of electrical charge stored on the piezoelectric stack and, therefore, it is possible to control the position of the valve needle relative to the valve seat. In this way, piezoelectric fuel injectors offer greater ability to meter the amount of fuel that is injected.
However, a number of disadvantages of direct-acting piezoelectric fuel injectors are also apparent. For example, one problem with these direct acting designs is that a relatively large and expensive piezoelectric actuator is needed to provide the energy needed to lift the needle. Furthermore, this type of actuator needs to get larger and/or more efficient as nozzle flow requirements and pressures increase. Another consideration with respect to large fuel injections is that the amount of needle lift is limited by the capabilities of the actuator (even if a hydraulic amplifier is used to try to alleviate this problem).
The invention relates to a fuel injector and to a method for operating a fuel injector so as to overcome or at least alleviate at least one of the above-mentioned problems in the prior art.